Separation of oleic acid from stearic and palmitic acids



April 5, 1955 c. w. HOERR 2,705,723

SEPARATION OF OLEIC ACID FROM STEARIC AND PALMITIC ACIDS Filed Aprii 21,1951 2 Shets-Sheet 1 80 I I C 50%0LEIC eozoualc ozouzlc f WEIGHT PERCENTOLElC-STEARIC ACID MIXTURES IN ANHYDROUS ACETONITRILE 0 l l l O 25 5O 75I00 WEIGHT PERCENT OLEIC-STEARIC ACID MIXTURES IN ACETONITRILECONTAINING 5% WATER //v VENTOR A r TORNEK April 5, 1955 c. w. HOERR2,705,723

SEPARATION OF OLEIC ACID FROM STEARIC AND PALMITIC ACIDS Filed April 21,1951 2 Sheets-Sheet' 2 V 80 I I I 'c.

50 f' 100% STEARIC j sozouzlc I I; OLEIC o 25 so 75 I00 WEIGHT PERCENTOLEIC-STEARIC ACID MIXTURES IN ACETONITRILE CONTAINING 20% WATER l I Iso sozounc 90%0LEIC IOO95OLEIC I o lNVENTOR:

o 5o I00 WEIGHT PEFICENT OLElC-STEARIC ACID MIXTURES IN ACETONITRILECONTAINING 25%WATER ATTORNEK United States Patent Charles W. Hoerr,Chicago, Ill., assignor to Armour and Company, Chicago, Ill., acorporation of Illinois Application April 21, 1951, Serial No. 222,252 7Claims. (Cl. 260-419) My invention relates to the separation of oleicfrom stearic and palmitic acids, and more particularly to the separationof these fatty acids by solvent crystallization.

Most of the common fats and oils are composed mainly of triglycerides ofoleic, stearic, and palmitic acids. The free fatty acids are readilyobtained from these fats and oils by subjecting the fatty materials toacid hydrolysis in the presence of naphthalene sulfonic acid, and thenseparating the glycerin from the fatty acids. Thus, there are availablea variety of commercial mixtures of free fatty acids, which aresubstantially composed of oleic, stearic, and palmitic acids.

Oleic acid is unsaturated and contains 18 carbon atoms, while stearicand palmitic acids are saturated and respectively contain 18 and 16carbon atoms. In the common mixtures of the free fatty acids stearic andpalmitic acids are found in roughly equal amounts. The total amount ofthese saturated fatty acids varies from about 20 to 50% by weight to thetotal amount of fatty acids present. In other words, the unsaturatedoleic acid is generally present in larger amounts than the saturatedfatty acids.

Since oleic acid due to its unsaturation has differing properties andtherefore different common uses from the saturated fatty acids, it isdesired to separate oleic acid from stearic and palmitic acids. It hasbeen found to be very difficult to produce a clean separation on acommercial basis by fractionation of the mixtures of the free fattyacids. Solvent extraction methods have also been found to be of littlevalue. At the present time, the separation is carried out by two solventcrystallization processes, in one of which acetone is employed as thesolvent and in the other of which methanol is employed.

Both the acetone and the methanol solvent crystallization processes forthe separation of oleic acid from stearic acids suffer from seriousdisadvantages. The fatty acids are highly soluble in both acetone andmethanol, and have very steep solubility curves in both solvents.Therefore, in carrying out these processes it is necessary to heat andthen refrigerate large volumes of solvent, which are costly operationsbecause of the spread in temperatures required to produce theseparation. Furthermore, the separation is not as efficient as might bedesired since the oleic acid begins to crystallize at the lowtemperatures necessary to complete the crystallization of the stearicand palmitic acids. In fact, it has been found impossible to obtain 90%or better separation between the saturated and unsaturated components ofthe fatty acids on a commercial scale with only one crystallization. Atpresent, if the saturated and unsaturated acids are desired in betterthan 90% purity, they must be subjected to recrystallization. Therefore,a need has long been felt for better crystallization solvents for use inseparating the fatty acids.

I have now discovered that acetonitrile has unexpectedly superiorproperties as a solvent for use in separating oleic acid from stearicand palmitic acids. Specifically, I have discovered that these fattyacids are all of limited solubility in acetonitrile, and that theseacids have very flat solubility curves separated by a relatively widetemperature range. These properties of acetonitrile mean that only arelatively small volume of solvent need be employed for separating agiven quantity of the fatty acids, and that the separation can becarried out over a narrow temperature range. Furthermore, cooling of thesolvent by refrigeration to crystallize the stearic and palrnitic acidsis not required since this crystallization occurs at or above ordinaryroom temperature.

In spite of these desirable properties of acetonitrile, it s probablynot a commercially practical solvent. This is true because oleic acid isnot completely miscible with acetonitrile in the desired commercialoperating range of from 10 to 50% by weight of fatty acids in thecrystallizing solution. When operating at fatty acid concentrationsabove 50%, the acetonitrile is actually dissolved in the fatty acids,and therefore the bulk of the crystallizing saturated acids will entrainportions of the solvent as well as absorbing quantities of the oleicacid. This results in a relatively poor separation between the saturatedand unsaturated fatty acids, and the entrained solvent must be removedfrom the product. On the other hand at concentrations below about 10%,the volume of the solvent required greatly increases the cost of theseparation. This is due to having to heat the larger volumes of solvent,and also to the larger equipment required.

When as little as 20% by weight of the fatty acids is oleic acid, a twophase system will exist between about 10% and 80% concentrations of thefatty acids in the acetonitrile. As the percentage of oleic acid isincreased, the two phase region exists over wider concentration ranges,until when the fatty acid mixture contains oleic acid the two phaseregion extends from concentra tions of about 5% to One of these phases,which will normally be the top layer, consists of a relatively in thefatty acids. Crystallization of the stearic and palmitic acids from thetwo phase system results in a much poorer separation between thesaturated and unsaturated acids than is obtained When the solution ishomogenous. Most of the saturated fatty acids are in the concentratedbottom layer which causes them to enextremely difficult operation.

Therefore, the problem is presented as to how to increase the solubilityof oleic acid in acetonitrile to such a point that oleic acid andacetonitrile will be completely miscible at all concentrations overtemperature ranges within which substantially all of the stearic andpalmitic acids can be crystallized. In other words, it is desired toselectively increase the solubility of oleic acid without solubilitycurves of these acids, as discussed above.

It is therefore an object of my invention a commercially feasible Morespecifically, it is an object of my invention to provide a means formodifying the solvent properties of acetonitrile to selectively increasethe solubility of an oleic acid over stearic and palmitic acids. It isalso an object of my invention to increase the temperature range withinwhich oleic acid and acetonitrile are completely miscible at allconcentrations. Further objects and advantages will appear as thespecification proceeds.

I have discovered that the acetonitri More specifically, I havediscovered that the separation of oleic acid from stearic acid can beremarkably improved by having present in the solvent from 3% to 25% ofwater to the combined weight of the acetonitrile and water, andpreferably between 5 and 20% by weight of water. This is a completelyunexpected result. All of the fatty acids are insoluble in water, and itis wellknown that the addition of water to other solvents for the fattyacids, and specifically for oleic, stearic, and palmitic acids,decreases the solubility of fatty acids in the solvent. It is thereforemy belief that I have discovered a unique phenomenon in that thepresence of certain critical amounts of water in acetonitrile increasesthe solubility of oleic acid therein. The effect of adding water toacetonitrile is also remarkable in that the solubility of palmitic andstearic acids therein are not appreciably changed. My investigationshave led me to believe that this phenomenon may be specific to systemsof. oleic, stearic, and palmitic acids in aqueous acetonitrile.

My invention is illustrated in the accompanying drawings, in whichFigure 1 shows the solubility of stearic and oleic acids and mixturesthereof in anhydrous acetonitrile; Fig. 2, the solubility curves of thefatty acids of Fig. 1 when the acetonitrile contains by Weight of water;Fig. 3, the solubility curves of the same fatty acids when theacetonitrile contains 20% water by weight and Fig. 4, the solubilitycurves of the same materials when the acetonitrile contains 25% water byweight.

Looking first at the phase diagram of Fig. 1, it can be seen that purestearic acid in anhydrous acetonitrile possesses a simple solubilitycurve consisting of a highly desirable broad flat region terminating ina quite abrupt change of slope at lower concentration. Oleic acid, onthe other hand, exists over a wide concentration range (approximately 5to 95%) as a heterogeneous system consisting of two immiscible liquids.This diagram also shows that as little as 20% oleic acid in stearic acidproduces this two phase region over the concentration range of the fattyacids in the solvent at which it would be desired to carry out thecrystallization process. Such a system, as discussed above isundesirable in solvent crystallization operations in that itnecessitates working at relatively great dilutions or results ininefficient separations at the higher concentrations.

Fig. 2 illustrates the unusual phenomenon resulting from the addition ofas little as 5% water by weight to the acetonitrile. This diagram showsthat when the oleic acid is present in the fatty acid mixture inconcentrations of not over 80%, that the oleic acid is com pletelymiscible at all concentrations down to temperatures below that at whichthe stearic acid crystallizes out of the solution. The improvement inthe solubility of oleic acid in acetonitrile becomes appreciable whenabout 3% water by weight is present in the solvent. As the concentrationof water increases from 3 to 5%, the solubility of the oleic acidincreases rapidly until the condition is reached illustrated in Fig. 2.At water concentrations in the solvent between 5% and 20% the solubilityof the oleic acid remains practically constant. At water concentrationsof about 20%, the system begins to revert to solubility conditionssimilar to those in anhydrous acetonitrile. The beginning of thisreversion is illustrated in Fig. 3 in which the solubility curves ofoleic and stearic acid mixtures are shown for acetonitrile contain-Between 20 and 25% water concentrations in the solvent, the solubilityof oleic acid decreases until at concentrations of 25% as illustrated inFig. 4, the presence of the water becomes of little value in modifyingthe system to improve the separation of the acids.

As illustrated in Figs. 1 through 4, the presence of water in theacetonitrile has practically no effect on the solubility of stearicacid. Palmitic acid has been found to have substantially identicalsolubility curves to that of stearic acid in anhydrous and aqueousacetonitrile, the curve for pure palmitic acid lying a few degrees lowerthan that for pure stearic acid. When stearic acid and palmitic acid areboth present in the solution, for all practical purposes theirsolubility curves are merged into one curve which has the same shape asthe curves for stearic acid and mixtures of stearic and oleic acid, butlies about 1 to 2 lower in general. Therefore, the phase diagrams shownin Figs. 1 to 4 can be taken as accurate for systems containing mixturesof oleic and stearic acid, mixtures of oleic and palmitic acid, andmixtures of oleic, stearic and palmitic acids.

In general, our solvent crystallization process employing a solventcomposed of acetonitrile and from 3 to 25% by weight of water andpreferably 5 to 20% by weight of water, can be carried out according towell-known procedures for similar solvent crystallization processes. Thefatty acids are first dissolved in the aqueous acetonitrile by slightlyelevating the temperature of the solvent. As shown by the accompanyingdiagram, an elevation of the temperature of the solvent to 60 C. will beadequate in practically all cases when separating commercially availablefatty acid mixtures to bring all of the fatty acids into an homogenoussolution. The exact temperature elevation necessary for varying mixturesof the fatty acids can easily be determined by consulting the phasediagrams.

After an homogenous solution has been produced by elevation of thetemperature of the solvent, the solvent is cooled to producecrystallization of the saturated fatty acids. Since it is not necessaryto cool the solution below about 30 C., refrigeration is not required tocool the solution and the cooling can be carried out by exposure of thesolution to room temperature. However, other cooling means can beemployed if it is desired to more accurately control the coolingtemperatures. After the crystallization has been completed, thecrystallized fatty acids can then be removed by filtration or otherequivalent separating processes. As will be apparent from the phasediagrams, it is important that the solution not be cooled to such apoint that the solubility of the oleic acid remaining in the solvent isso decreased as to produce a two phase system. I have found that if thefiltration is carried out at temperatures at which the filtrate containstwo immiscible liquid systems, that some of the oleic acid will bepushed out of solution and entrapped in the precipitate by adsorption,thus decreasing the purity of the product. I have also discovered thatthe optimum lower temperature in the cooling step and in the filtrationstep is preferably not over 5 C. above the temperature at which thematerial in the filtrate forms into immiscible liquid systems. Since mymethod of solvent crystallization when correctly carried out results ina or better separation between the saturated and unsaturated acids, thefiltrate will consist of oleic acid in excess of 90%. As illustrated inFigs. 2 and 3, when operating with between 5 and 20% water in thesolvent and at fatty acid concentrations in the solvent of greater than10%, the filtrate can be expected to begin to contain immiscible layersat temperatures between 30 C. and 40 C. I have found, as indicatedabove, that when the lower cooling temperature and the filtrationtemperature is more than 5 above the temperature at which the immisciblesystems begin to form, that a greater proportion of the saturated acidswill be left in the filtrate, and thereby decrease the cleanness of theseparation. In practical operation of my process, I have found that thepreferred lower cooling temperatures and filtration temperatures arebetween about 34 to 36 C., and that excellent results are obtained whena temperature of 35 C. is employed. The exact minimum temperature for agiven concentration of oleic acid in the filtrate can easily bedetermined experimentally or by consulting the phase diagrams.

After the separation of the crystallized saturated fatty acids, theoleic acid can easily be recovered from the filtrate by sending thefiltrate to a steam heated stripping still in which the solvent isremoved. The oleic acid recovered from this operation is found to be ofhigh grade and purity when the prior crystallization and filtrationsteps have been carried out according to our preferred method.

If desired, the stearic and palmitic acids can be separated bywell-known procedures, such as fractionation, etc.

My process can be applied to a wide variety of fatty materials. However,I prefer to utilize as starting materials mixtures of free fatty acidscontaining not over 80% oleic acid in combination with stearic andpalmitic acids. Among the fatty acid mixtures which meet thesespecifications are: Palm oil (50% oleic-50% palmiticstearic), tallow(50% oleic50% palmitic-stearic), lard (65% oleic-35% stearic-palmitic),cottonseed oil (75% oleic25% palmitic-stearic) and peanut oil (80%oleic- 20% palmitic-stearic). In order to better illustrate my pfocess Iwish to set forth the following illustrative examp es:

Example I of a fatty acid mixture consisting of 47% stearic acid and 53%oleic acid was dissolved in 9 parts of solvent consisting of 90%acetonitrile and 10% water at about 40 C. After cooling the solution to31 C., the solid matter which precipitated was separated from thesolution by filtration at 31 C. This precipitate was One part separatedfrom the solution by filtration at 31 C. This precipitate amounted to49% of the original fatty acid mixture and consisted of 97% stearic acidand 3% oleic acid. The residue obtained by evaporating the solvent fromthe filtrate amounted to 51% of the original fatty acid mixture andconsisted of 96% oleic acid and 4% stearic acid.

Example II One part of a fatty-acid mixture containing 55% stearic acidand 45% oleic acid was dissolved in two parts of solvent consisting of80% acetonitrile and water at about 55 C. This solution was cooled to C.and the solid matter which precipitated was separated from the solutionby filtration at 35 C. The precipitate amounted to 54% of the originalfatty-acid mixture and consisted of 98% stearic acid and 2% oleic acid.The residue obtained by evaporation of the solvent from the filtrateamounted to 46% of the original fatty-acid mixture and consisted of 96%oleic acid and 4% stearic acid.

Example 111 One part of a regular commercial tallow fatty-acid feedstockconsisting of 45% saturated fatty acids (essentially palmitic andstearic acids in the ratio 60:40) and 55% unsaturated acids (essentiallyoleic acid) was dissolved in two parts of solvent consisting of 80%acetonitrile and 20% water at 45 C. The solution was cooled to 35 C. andfiltered at this temperature. The solid material which precipitatedamounted to of the original feedstock and consisted of 96% saturatedacids and 4% unsaturated acids. The residue obtained by evaporation ofthe solvent from the filtrate amounted to 60% of the original feedstockand consisted of 92% unsaturated acids and 8% saturated acids.

Example IV One part of the tallow acid feedstock of Example III wasdissolved in three parts of solvent consisting of 95 acetonitrile and 5%water at C. This solution was cooled to 35 C. and the solid matter whichprecipitated was removed by filtration at this temperature. Thisprecipitate amounted to 43% of the original feedstock and consisted of98.5% saturated acids and 1.5% unsaturated acids. The residue obtainedby evaporation of the solvent from the filtrate amounted to 57% of theoriginal feedstock and consisted of 95% unsaturated acids and 5%saturated acids.

Example V One part of the tallow acid feedstock of Example III wasdissolved in two parts of solvent consisting of 95 acetonitrile and 5%water at 45 C. This solution was cooled to 35 C. and the solid matterwhich precipitated was removed by filtration at this temperature. Thisprecipitate amounted to 40% of the original feedstock and consisted of96% acids. The residue obtained by evaporation of the solvent from thefiltrate amounted to 60% of the original feedstock and consisted of 92%unsaturated acids and 8% saturated acids.

As pointed out above, it is important to have the lower coolingtemperatures and the filtration temperature of the solvent not over 5above the temperature at which the filtrate begins to contain immisciblelayers, and preferably at a temperature just above the temperature atwhich the material in the filtrate forms an immiscible liquid system. Inother words, the separation obtained becomes progressively sharper up tothe temperature at which the immiscible systems begin to form in thefiltrate. The criticalness of these temperatures is illustrated by thefollowing example in which the separation of Example II was carried outat a higher filtration temperature.

Example VI One part of a fatty acid mixture containing 54% stearic acidand 46% oleic acid was dissolved in 2 parts of solvent consisting of 80%acetonitrile and 20% water at about 50 C. This solution was cooled to 40C. and the solid matter which precipitated was separated from thesolution by filtration at 40 C. The precipitate amounted to 45 of theoriginal fatty acid mixture and consisted of 90% stearic acid and 10%oleic acid. The residue obtained by evaporation of the solvent from thefiltrate amounted to of the original fatty acid mixsaturated acids and4% unsaturated turg and consisted of 77% oleic acid and 23% stearic actAlthough my invention has been described in connection with certainspecific embodiments, it will be apparent that modifications and changescan be made without departing from the spirit and scope of my invention.

I claim:

1. In a solvent crystallization process for separating oleic acid fromstearic and palmitic acids wherein a mixture of said fatty acids isdissolved in acetonitrile at a slightly elevated temperature and thenthe solution is cooled until crystallization of the stearic and palmiticacids occur, the improvement comprising having present in said solutionfrom 3 to 25% by weight of water to the total weight of the water andacetonitrile during said cooling step to selectively increase thesolubility of the oleic acid in the acetonitrile and thereby greatlyincrease the temperature range within which oleic acid and acetonitrileare completely miscible at all concentrations.

2. In a solvent crystallization process for separating oleic acid fromstearic acid wherein a. mixture of said fatty acidsis dissolved inacetonitrile at a slightly elevated temperature and then the solution iscooled until crystallization of the stearic acid occurs, the improvementcomprising having present in said solution from 3 to 25 by weight ofwater to the total weight of the water and acetonitrile during saidcooling step to selectively increase the solubility of the oleic acid inthe acetonitrile the temperature range within at all concentrations.

3. In a solvent crystallization process for separating oleic acid fromstearic and palmitic acids wherein a mixture of said fatty acids isdissolved in acetonitrile at a slightly elevated temperature and thenthe solution is cooled until crystallization of the stearic and palmiticacids occur, the improvement comprising having present in said solutionfrom 5 to 20% total weight of the water and acetonitrile during saidcooling step to selectively increase the solubility of the oleic acid inthe acetonitrile and thereby greatly increase the temperature rangewithin which oleic acid and acetonitrile are completely miscible at allconcentrations.

4. In a solvent crystallization process for separating oleic acid fromstearic acid wherein a mixture of said fatty acids is dissolved inacetonitrile at a slightly elevated temperature and then the solution iscooled until crystallization of the stearic acid occurs, the improvementcomprising having present in said solution from 5 to 20% by weight ofwater to the total weight of the water and acetonitrile during saidcooling step to selectively increase the solubility of the oleic acid inthe acetonitrile and thereby greatly increase the temperature rangewithin w 5. In a solvent crystallization process for separating oleicacid from stearic and palmitic acids, the steps of a fatty acid mixturecontaining oleic, stearic, and palmitic acids in aqueous acetonitrilecontaining from 3 to 25% water by heating said solvent until anhomogenous solution is produced, cooling the solution to a temperatureat which the stearic and palmitic acids crystallize out of solution,said cooling temperature being above but not over 5 C. above thetemperature at which the oleic acid and solvent separate into twoimmiscible layers, and filtering the solution to remove the crystallizedstearic and palmitic acids at a temperature above but not over 5 C.above the temperature at which the filtrate begins to contain twoimmiscible portions.

6. In a solvent crystallization process for separating oleic acid fromstearic and palmitic acids, the steps of dissolving a fatty acid mixturecontaining oleic, stearic, and palmitic acids in aqueous acetonitrilecontaining from 5 to 20% water by heating said solvent until anhomogenous solution is produced, cooling the solution to a temperatureat which the stearic and palmitic acids crystallize out of solution,said cooling temperature being above but not over 5 C. above thetemperature at which the oleic acid and solvent separate into twoimmiscible layers, and filtering the solution to remove the crystallizedstearic and palmitic acids at a temperature above but not over 5 C.above the temperature at which the filtrate begins to contain twoimmiscible portions.

7. In a solvent crystallization process for separation of oleic acidfrom stearic and palmitic acids, the steps cornby weight of water to thefrom 10 to 50% by weight of a mixture containing oleic, stearic, andpalmitic acids in aqueous acetonitrile containing from 5 to 20% water byweight by heating said solvent, said fatty acid mixture containing lessthan about 80% oleic acid by weight, cooling said solution to atemperature between about 34 to 36 C. to crystallize said stearic andpalmitic acids, and filtering said solution to remove the crystallizedmixture at a temperature between about 34 and 36 C.

References Cited in the file of this patent UNITED STATES PATENTS GroteApr. 12, 1938 prising dissolving 8 2,200,391 Freeman II May 14, 19402,313,636 Freeman Mar. 9, 1943 2,320,738 Jenkins June 1, 1943 OTHERREFERENCES Ferris et al., Ind. and Eng. Chem. 23, N0. 7 (1931), pp.753-755.

Kleinsmith et al., Ind. and Eng. Chem., No. 6 (1943), 10 pp. 674-676.

1. IN A SOLVENT CRYSTALLIZATION PROCESS FOR SEPARATING OLEIC ACID FROMSTEARIC AND PALMITIC ACIDS WHEREIN A MIXTURE OF SAID FATTY ACIDS ISDISSOLVED IN ACETONITRILE AT A SLIGHTLY ELEVATED TEMPERATURE THEN THESOLUTION IS COOLED UNTIL CRYSTALLIZATION OF THE STEARIC AND PALMITICACIDS OCCUR, THE IMPROVEMENT COMPRISING HAVING PRESENT IN SAID SOLUTIONFROM 3 TO 25% BY WEIGHT OF WATER TO THE TOTAL WEIGHT OF THE WATER ANDACETONITRILE DURING SAID COOLING STEP TO SELECTIVELY INCREASE THESOLUBILITY OF THE OLEIC ACID IN THE ACETONITRILE AND THEREBY GREATLYINCREASE THE TEMPERATURE RANGE WITHIN WHICH OLEIC ACID AND ACETONITRILEARE COMPLETELY MISCIBLE AT ALL CONCENTRATIONS.